Priorly, such a formed article 1 was obtained through two forming processes, that is, a process in which spline teeth are formed on large-diameter drum part 1c and a process in which spline teeth are formed on small-diameter drum part 1b.
However, since this procedure causes man-hours to increase and since deviations may occur in the concentricity of the upper and lower teeth 2 and 3, methods of simultaneously forming teeth 2 and 3 by a single forming process have been proposed.
FIG. 10 shows the structure of a die assembly for this type of simultaneous forming in which the inner periphery of die 4 is comprised of two steps in the vertical direction, that is, a small-diameter part 5 and a large-diameter part 6 disposed above small-diameter part 5 and wherein forming teeth 5a and 6a are formed on the inner periphery of each part.
On the other hand, punch 7 has a small-diameter punch part 8 and a continuing large-diameter punch part 9 formed integrally, and forming teeth 8a and 9a are formed on the outer periphery of the respective punch parts.
With the above arrangement, a to-be-processed article 10, which is a cup-shaped, press-formed sheet-metal article, having two step portions of a large and small diameter, respectively, prior to forming, is fitted onto punch 7 and then pressed into die 4 as shown in FIGS. 11(a) and (b) so that the forming teeth 5a, 8a, 6a, and 9a engage in the axial direction respectively. The peripheral walls of the to-be-processed article 10 is thereby ironed in the axial direction and a stepped, cup-shaped formed article 1, having spline teeth 2 and 3 of a protruded and indented form formed on the inner and outer peripheries, is thus obtained in one shot.
However, with the above-described simultaneous forming method, punch 7 is lowered at a constant pressurizing force to iron large-diameter drum part 1c and small-diameter drum part 1b of formed article 1 simultaneously. Thus in the forming process, the ironed material on the peripheral wall of small-diameter drum part 1b is pushed outwards (made to flow outwards) at step portion 1a of formed article 1 as shown by the arrow in FIG. 11(a) and bulges in the form of excess thickness A at the upper side of step 1a. Furthermore, as shown in on enlarged manner in FIG. 11(b), the excess thickness A is pressed by the step 7a of punch 7, causing a defect B, which may be a gouge, shrinkage cavity, or entrainment, to occur and cause a portion of the article to become thin and weak in strength.
The bulge due to the excess thickness that forms at step 1a is especially significant when step 1a is to be formed to be perpendicular to the peripheral walls, and such a bulge must be cut off if the formed article is to be used in a clutch drum. Thus if the formed article is to be used as it is in a clutch drum and without performing a cutting process, the step 1a between small-diameter drum part 1b and large-diameter drum part 1c must be inclined as shown in FIG. 11(b) so that the bulge due to excess thickness A will not protrude towards the inner side of large-diameter drum part 1c. It was therefore difficult to form step 1a to be perpendicular to the peripheral walls.
Thus with the prior art, it was difficult to make the diameter difference D between the upper and lower parts at step 1a large since this lead to the increasing of the axial length of step 1a, and thus to the increasing of the axial length of the clutch drum, which in turn lead to the increasing of the size of the entire clutch device.
Upon examining the causes by which excess thickness A and defect B occur, the present inventor found that since a gap C is formed between formed article 1 and punch step 7a in consideration of the elongation of small-diameter drum part 1b, the material that flows upward in the axial direction by the iron-forming of small-diameter drum part 1b bulges at step 1a and is then pressed by punch step 7a to give rise to excess thickness A and defect B.
Thus in order to prevent a gap from being formed between the step portions of the processed article and the punch, the present inventor provided the punch with an arrangement in which it was split into a first punch for forming small-diameter part 1b and an outer peripheral punch for forming large-diameter drum part 1c, and by maintaining the ironing pressure of the first punch after forming large-diameter drum part 1c and controlling the ironing pressure of the outer peripheral punch to be lower than the normal ironing pressure in ironing the small-diameter drum part 1b so that the ironing pressure of the outer peripheral punch will not act on the step of the processed article or the small-diameter drum part, was able to obtain a processed item with which neither excess thickness A nor defect B occurred at the step portion.
The present invention was made based on the above-described problems of the prior art and on the findings of the inventor, and the objects thereof include the provision of a method for forming spline teeth by ironing on a stepped, press-formed sheet-metal article and a provision for a sheet metal clutch drum formed by ironing by said method, with which excess thickness does not occur at the step portions in the process of iron-forming the spline teeth on a plurality of step portions in one shot and with which forming can be performed without geometrical restrictions such as in the differences in diameter between upper and lower parts, inclination of the step portion, etc.